Natural gas, found in deposits in the earth, is an abundant energy resource. For example, natural gas commonly serves as a fuel for heating, cooking, and power generation, among other things. The process of obtaining natural gas from an earth formation typically includes drilling a well into the formation. Wells that provide natural gas are often remote from locations with a demand for the consumption of the natural gas.
Thus, natural gas is conventionally transported large distances from the wellhead to commercial destinations in pipelines. This transportation presents technological challenges due in part to the large volume occupied by a gas. Because the volume of an amount of gas is so much greater than the volume of the same number of gas molecules in a liquefied state, the process of transporting natural gas typically includes chilling and/or pressurizing the natural gas in order to liquefy it. However, this contributes to the final cost of the natural gas and is not economical.
Formations that include small amounts of natural gas may include primarily oil, with the natural gas being a byproduct of oil production that is thus termed associated gas. In the past, associated gas has typically been flared, i.e., burned in the ambient air. However, current environmental concerns and regulations discourage or prohibit this practice.
Further, naturally occurring sources of crude oil used for liquid fuels such as gasoline, jet fuel, kerosene, and diesel fuel have been decreasing and supplies are not expected to meet demand in the coming years. Fuels that are liquid under standard atmospheric conditions have the advantage that in addition to their value, they can be transported more easily in a pipeline than natural gas, since they do not require liquefaction.
Thus, for all of the above-described reasons, there has been interest in developing technologies for converting natural gas to more readily transportable liquid fuels, i.e. to fuels that are liquid at standard temperatures and pressures. One method for converting natural gas to liquid fuels involves two sequential chemical transformations. In the first transformation, natural gas or methane, the major chemical component of natural gas, is reacted with oxygen to form syngas, which is a combination of carbon monoxide gas and hydrogen gas. In the second transformation, known as the Fischer-Tropsch process, carbon monoxide is reacted with hydrogen to form organic molecules containing carbon and hydrogen. Those molecules containing only carbon and hydrogen are known as hydrocarbons. Those molecules containing oxygen in addition to carbon and hydrogen are known as oxygenates. Hydrocarbons having carbons linked in a straight chain are known as aliphatics and are particularly desirable as the basis of synthetic diesel fuel.
The Fischer-Tropsch process is commonly facilitated by a catalyst. Catalysts desirably have the function of increasing the rate of a reaction without being consumed by the reaction. Common catalysts for use in the Fischer-Tropsch process contain at least one metal from Groups 8, 9, or 10 of the Periodic Table (in the new IUPAC notation, which is used throughout the present specification). The molecules react to form hydrocarbons while confined on the surface of the catalyst. The hydrocarbon products then desorb from the catalyst and can be collected. H. Schulz (Applied Catalysis A: General 1999, 186, p 3) gives an overview of trends in Fischer-Tropsch catalysis.
The catalyst may be contacted with synthesis gas in a variety of reaction zones that may include one or more reactors. Common reactors include packed bed (also termed fixed bed) reactors, slurry bed reactors, and fluidized bed reactors. Originally, the Fischer-Tropsch synthesis was carried out in packed bed reactors. These reactors have several drawbacks, such as poor temperature control, that can be overcome by gas-agitated slurry reactors or slurry bubble column reactors. Gas-agitated multiphase reactors sometimes called “slurry reactors” or “slurry bubble columns,” operate by suspending catalytic particles in liquid and feeding gas reactants into the bottom of the reactor through a gas distributor, which produces small gas bubbles. As the gas bubbles rise through the reactor, the reactants are absorbed into the liquid and diffuse to the catalyst where, depending on the catalyst system, they are typically converted to gaseous and liquid products. The gaseous products formed enter the gas bubbles and are collected at the top of the reactor. Liquid products are recovered from the suspending liquid by using different techniques like filtration, settling, hydrocyclones, magnetic techniques, etc. Gas-agitated multiphase reactors or slurry bubble column reactors (SBCRs) inherently have very high heat transfer rates; therefore, reduced reactor cost and the ability to remove and add catalyst online are principal advantages of such reactors in Fischer-Tropsch synthesis, which is exothermic. Sie and Krishna (Applied Catalysis A: General 1999, 186, p. 55) give a history of the development of various Fischer Tropsch reactors.
Typically the Fischer-Tropsch product stream contains hydrocarbons having a range of numbers of carbon atoms, and thus having a range of molecular weights. Thus, the Fischer-Tropsch products produced by conversion of synthesis gas commonly contains a range of hydrocarbons including gases, liquids and waxes. It is highly desirable to maximize the production of high-value liquid hydrocarbons, such as hydrocarbons with at least 5 carbon atoms per hydrocarbon chain (C5+hydrocarbons).
The composition of a catalyst influences the relative amounts of hydrocarbons obtained from a Fischer-Tropsch catalytic process. Cobalt metal is particularly desirable in catalysts used in converting natural gas to hydrocarbons suitable for the production of diesel fuel. Further, iron, nickel, and ruthenium have been used in Fischer-Tropsch catalysts. Nickel catalysts favor termination and are useful for aiding the selective production of methane from syngas. Iron has the advantage of being readily available and relatively inexpensive but the disadvantage of a water-gas shift activity. Ruthenium has the advantage of high activity but is quite expensive.
One of the limitations of a Fischer-Tropsch process is that the activity of the catalyst will, due to a number of factors, deteriorate over time. The catalyst can be regenerated by exposure to a stream of hydrogen or some other gas. One preferred technique for the regeneration of catalyst takes place at temperature that is well above the temperature maintained within the reactor. Therefore, it is necessary to provide a source for increasing the temperature of the catalyst for regeneration and maintaining that temperature during the regeneration process. Maintaining the catalyst at the desired temperature during regeneration is further complicated by the regeneration process being slightly endothermic, thereby acting to decrease the temperature of the catalyst during regeneration.
Several methods are presently used to supply the heat needed to achieve and maintain the temperature of the catalyst during the regeneration process. These methods generally include disposing a heat source within, or outside of, a vessel containing the catalyst. This heat source may be an electric resistance heater or a heating coil circulating a high temperature fluid, such as steam. Both of these methods may potentially be very expensive to install and maintain. For example, high pressure steam used for heating is typically circulated through steam coils that must be constructed from high strength materials, which are normally expensive, and electrical resistance based heating often results in very high operational costs. These types of devices may also create a barrier to the free flow of material because the heat source is disposed directly in the vessel.
Thus, there remains a need in the art for methods and apparatus to improve the efficiency of achieving and maintaining the temperature for a catalyst regeneration process. Therefore, the embodiments of the present invention are directed to methods and apparatus for constructing and operating a regeneration system that seek to overcome these and other limitations of the prior art.